The present invention relates to the recovery of molybdenum from solution. Here, the more specific use of the process is to recover molybdenum as ammonium octamolybdate in a form that is substantially free from vanadium.
Many hydrocarbon feedstocks contain high percentages of metals as contaminants, e.g., iron, nickel, or vanadium. Because of the growing shortages of petroleum in the world refiners are forced to use feedstocks that contain larger amounts of these metals. Once they are in the feed, they tend to deposit on the surfaces of and/or in the interstices between hydroprocessing catalysts which contain catalytic metals (e.g., cobalt, nickel, molybdenum, or tungsten). When the hydroprocessing catalysts no longer give the desired conversion rates (due to plugging of the catalyst pores by coke or the contaminant metals or due to reactor plugging), they are replaced and subsequently disposed of (and are called "spent catalysts"). Because of environmental and economic considerations, it is therefore desirable to recover the heavy metal values from the spent catalyst, leaving the catalyst support in an environmentally benign form.
The recovery of metals such as molybdenum and vanadium from spent catalysts is nothing new of course, as indicated by U.S. Pat. No. 4,500,495, issued Feb. 19, 1985 to Hubred et al; U.S. Pat. No. 4,554,138, issued Nov. 19, 1985 to Marcantonio; as well as U.S. Pat. Nos. 4,434,140 and 4,434,141, both issued Feb. 28, 1984 to Hubred et al, just to name a few which are totally incorporated by reference. However, these patents have never addressed the separation of molybdenum from vanadium into a substantially pure form.
The recovery of molybdenum from various sources has been described in the prior art [see U.S. Pat. No. 3,458,277, issued July 29, 1969 to Platzke et al and U.S. Pat. No. 3,957,946, issued May 18, 1978 to Ronzio et al; and J. Litz, "Solvent Extraction of W, Mo, and V: Similarities and Contrasts", Extractive Metallurgy of Refractory Metals, 69-81 (1980)]. See also, U.S. Pat. No. 3,455,677, issued Aug. 1, 1972 and U.S. Pat. No. 3,681,016, issued July 15, 1969 to John E. Litz. U.S. Pat. No. 3,455,677 involves evaporative crystallization of and precipitation molybdenum in a process to recover copper and molybdenum from ore concentrates. This patent describes two methods of forming a molybdenum compound. The first is by evaporative crystallization to force molybdenum out of solution at its solubility limit and the second is a high temperature polymerization which causes molybdenum precipitation. U.S. Pat. No. 3,681,016 involves the separation and recovery of molybdenum and rhenium values from solution.
Recovery of vanadium has also been described in the prior art. See U.S. Pat. No. 4,544,533, issued Oct. 1, 1985 and U.S. Pat. No. 4,551,315, issued Nov. 5, 1985 to P. J. Marcantonio, and U.S. Pat. No. 4,432,949 to G. L. Hubred et al. The patents to Marcantonio involve the recovery of vanadium from an aqueous ammonium bicarbonate strip solution and a subsequent precipitation. They are incorporated by reference. The patent to Hubred et al precipitates vanadium then removes nickel, cobalt, and molybdenum by serial ion exchange.
However, the prior art has not addressed or adequately solved the problem of vanadium contamination of molybdenum when evaporative crystallization or a chemical precipitation method is used.
It is an object of this invention to chemically precipitate molybdenum from a mixed solution of molybdenum and vanadium. It is a further object of this invention to precipitate molybdenum while rejecting at least 80% of the associated vanadium.